Several approaches have been applied in the past to efforts to develop practical fire-retardant paints. Generally, these approaches have involved the incorporation of chemicals in the paint, to function in varying ways to protect the substrate, e.g., materials have been used which evolve non-flammable gases to blanket the surface, excluding oxygen from the fuel; highly hydrated materials have been used, which evolve water vapor to cool the substrate; vitreous materials, which envelop the substrate in a non-combustible glass; and, materials which intumesce, forming a non-flammable insulative foam which directs the heat away from the substrate. These approaches provide only temporary protection for the substrate, but the ignition and comsumption of the substrate will be substantially slowed, so that time is allowed for the removal of persons and goods and for fire-fighting equipment to be summoned. This delay in spread-of-the-fire will therefore reduce the overall damage which will occur in a given period of time. Intumescent coatings are generally thought to offer protection for a longer period of time than do any of the other approaches.
An intumescent coating is one that has ingredients that react, upon heating, to form large amounts of an incombustible or substantially incombustible residue, that is expanded to a cellular foam having good thermal insulating properties. Generally such coatings have been developed and used as paints for application to solid surfaces such as wood surfaces. Upon exposure to high temperature or to a flame, the paint forms a heat-insulating layer on the coated surface, to protect it from heat. The principle has also been applied to the formulation of electrical insulation for coating electrical conductors, and to films.
Ideally the heat insulating layer should prevent or retard energy transfer, so that the coated substrate does not become sufficiently hot to deteriorate, i.e., reach its kindling point, melt, or the like.
In intumescent paints as often formulated in the past, there have been three basic types of ingredients used to produce the foam layer:
1. a source of carbon (the carbonific);
2. a phosphorus-releasing material (the catalyst); and
3. a source of non-flammable gases (the blowing agent).
As the intumescent paint is subjected to increased temperatures the catalyst decomposes, yielding phosphoric acid. Often, ammonium orthophosphate has been used as the catalyst, although the utility of an amine catalyst and amide phosphate reaction products such as melamine phosphate and polyphosphorylamide has also been described in the patent literature, see for example U.S. Pat. No. 3,969,291. To be effective, a catalyst must contain a high phosphorus content and decompose to yield phosphoric acid.
Upon release of the phosphoric acid from the catalyst and at an elevated temperature, reaction or association occurs with the carbonific. The carbonific is generally chosen from the carbohydrates, proteins or polyfunctional alcohols; e.g., starch, casein or pentaerythritol. To be effective the carbonific must contain a large number of "phosphorus-esterifiable" sites, a large percentage of carbon, and must decompose at a higher temperature than the catalyst.
Following reaction between the phosphoric acid and the carbonific, the resultant "ester" begins to decompose, producing a large volume of carbon, additional water, carbon monoxide, and non-flammable gases, and releasing the acid for further esterification. In general, the ester begins to decompose at a temperature significantly lower than the initial "non-esterified" carbonific.
Simultaneously with decomposition of the ester, the blowing agents begin to decompose, yielding large volumes of non-flammable gases. These gases cause the carbonaceous residue from the "ester" to bubble and foam, to form a thick insulative mat. Often two blowing agents, with slightly differing decomposition temperatures, are used, in order to extend the length of gas release. This results in greater foam heights. One blowing agent is usually chosen from the amides or amines, e.g., dicyanidiamide, urea, melamine or guanidine, while the other, lower temperature blowing agent is normally a chlorinated paraffin such as Chlorowax 70. "Chlorowax 70" is the registered trademark of Diamond Shamrock for a resinous chlorinated hydrocarbon product having the formula C.sub.24 H.sub.28 Cl.sub.22, a chlorine content of 70% by weight, a flash point, closed cup, of none under 400.degree. F., and a specific grafity of 1.66 at 25.degree. C.
There is considerable overlapping of the reagent functions. For example, the catalyst releases gaseous materials such as ammonia, which may assist in blowing, although gaseous release from this source is nearing completion before the bulk of the carbon is formed. In another case, the blowing agents provide some additional carbon residue. However, these side benefits contribute only slightly, it is believed, to the end result.
In addition to the reactants discussed above, some formulations utilize a resinous material; such as, for example, a urea-or melamine-formaldehyde resin. This material melts to form a film or "skin" over the forming foam. This "skin" retains the gaseous materials within the foam, and tends to produce more continuous and uniform level of intumescence. When thermoplastic resinous binders are used, they generally assist in this function.
In summary then, the mechanism of intumescence consists of several steps, which may occur in this general order:
1. the catalyst decomposes to form phosphoric acid;
2. the resultant acid reacts with the carbonific;
3. the phosphated carbonific decomposes to form a large volume of foamable carbon and gas, then releases the acid;
4. the resinous material melts to form a film or "skin" over the foamable carbon, and
5. the blowing agents release gases (probably simultaneously with 3, above) which further cause the carbon to foam, while the "skin" assists in retaining the gas in place in the foamed layer, thus forming a thick, highly effective thermal insulation layer.
Ammonium polyphosphate releases a greater quantity of phosphoric acid, when used in a fire-retardant paint coated on a wood surface, and heated, than any other known catalyst. It is therefore a preferred material for use, and is known as an excellent fire-retardant additive for an intumescent coating composition. For example, British Pat. No. 1,171,491 discloses that intumescent coating compositions containing a suitable amount of ammonium polyphosphate achieved a Class A fire rating in the ASTM E-84-50T fire retardant test when applied to a panel of yellow poplar heartwood with a coverage of about 150 ft.sup.2 /gal. U.S. Pat. No. 3,969,291 also comments upon its use.
Ammonium polyphosphates used as fire-retardant additives in conventional intumescent paint compositions are represented by the general formula: EQU H.sub.(n-m)+2 (NH.sub.4).sub.m P.sub.n O.sub.3n+1
wherein n is an integer having an average value greater than 10, and the maximum value of m is equal to n+2, and m/n has an average value between 0.7 and 1.1. Commercial ammonium polyphosphate is defined as having a water solubility of about 5 grams or less per 100 cc of water, evaluated by slurrying 10 grams of the solids in 100 cc of water for 60 minutes at 25.degree. C.
Although the ammonium polyphosphates represented by the above formula are said to be "substantially water-insoluble", commercial ammonium polyphosphate is soluble in water to some extent since, in actuality, the solubility of the 10 g of ammonium polyphosphate is about 1 to 5 g per 100 cc of water measured in the above-described manner. Hence, a paint composition containing this ammonium polyphosphate may not have sufficient water-resistance, although it may be superior in these regards to a paint composition containing conventional water-soluble ammonium phosphates, such as ammonium dihydrogenphosphate.
However, even the limited solubility it possesses would generally contraindicate its use in treating fabrics, particularly materials such as canvas that is intended to be exposed to the weather.
While there are fire-retardant fabric treating compositions available, they generally are not as effective as is desirable, requiring too long a period for the formulation of a thermal insulating barrier to be practical, and offering too little by way of thermal insulation. In particular, nylon and Dacron polyester fabrics, when treated with prior art flame retardants, are prone to melt and drip, so that a human wearer of such a treated fabric could suffer burns from the material that is intended to be protective.
Historical surveys of treating fabric to impart fire retardance coupled with weather resistance may be found in:
1. Textile World, 93, 90 (1943).
2. Flameproofing Textile Fabrics, Am. Chem. Soc. Monograph 104, Reinhold, N.Y., 1947.
3. The Chemistry and Uses of Fire Retardants, J. W. Lyons, Wiley-Interscience, 1970, pp. 165-240 (For paints, see pp. 256-272).
4. Review of Textile Progess, published annually since 1948; see the sections on "Fireproofing" and "Flameproofing".